Current speed of my workers is reported as 0.0 kH/s instead of about 500 MH/s.
They are connected to litecoinpool and work as usual, just the speed displayed at the site is zero.
What can be a cause of it?
If the pool reports a speed of zero it means that your miner is not submitting shares. This can happen for a variety of reasons, ranging from hardware failure to software misconfiguration. Since there are many possible causes, it is hard to debug this issue without further insight. You should try to look for error messages, but unfortunately many ASIC interfaces are not very helpful in this regard.
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Wrong. Both my miners are submitting shares. For several days "24-hour" speed is displayed correctly and rewards are growing, yet "Speed" is displayed as zero. "Stale shares" value is around 0.4% and "Invalid shares" is 0.00%.
The "24-hour" speed value alone is not a good indicator of work, because in case of failure I will know about it late. Occasionally I perform thermal tuning - the "Speed" display at the site is crucial for that. The values reported by miners' soft are usually misleading.